The scorecard is in from boxing's "Clash in Cotai" this weekend in Macau. In what may have been the final fight of his career, Filipino champ Manny Pacquiao won a unanimous decision over American Brandon Rios. The other winner was Macau, which gained prestige on the global sports and entertainment scene

The biggest loser? That would be Uncle Sam, thanks to U.S. tax rates that push business overseas and hamper the economy in Las Vegas and beyond.

Mr. Pacquiao's previous 14 fights were in the U.S., but this time the former eight-division world champion specifically requested not to fight there. His stated reason: America's top federal marginal tax rate of 39.6%. "It all comes down to m-o-n-e-y," says promoter Bob Arum. "Manny can go back to Las Vegas and make $25 million, but how much of it will he end up with—$15 million?"

Since 2006 the "Pac-Man" had fought only in Nevada and Texas, where there are no state income taxes. This year new and higher federal rates made any U.S. site a "no-go," said Mr. Pacquiao's financial adviser Michael Koncz in February. ...

The "Clash in Cotai" is the latest, punchiest reminder that business can often float like a butterfly—especially when taxes sting like a bee.